LTE uses OFDM in the DL and DFT-spread OFDM in the UL. The basic LTE DL physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1A where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE DL transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten (#0-#10) equally-sized subframes of length Tsubframe=1 ms, as illustrated in FIG. 1B. The resource allocation in LTE is typically described in terms of RBs, where a RB corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent RB in time direction (1.0 ms) is known as a RB pair. RBs are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
The notion of virtual resource blocks (VRB) and physical resource blocks (PRB) has been introduced in LTE. The actual resource allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain; thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
DL transmissions are dynamically scheduled, i.e., in each subframe (see FIG. 1B) the base station (eNB) transmits control information about to which terminals data is transmitted and upon which RB the data is transmitted, in the current DL subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The DL subframe also contains common reference symbols (CRS), which are known to the receiver and are used for coherent demodulation of e.g. the control information. A DL system with CFI=3 OFDM symbols as control in a control region is illustrated in FIG. 2.
The 3GPP Rel-10 specifications for LTE/E-UTRAN have been standardized, supporting CC bandwidths up to 20 MHz (which is the maximal LTE Rel-8 carrier bandwidth). An LTE Rel-10 operation wider than 20 MHz is possible and appear as a number of LTE CCs to an LTE Rel-10 terminal. A straightforward way to obtain bandwidths wider than 20 MHz is by means of CA. CA implies that an LTE Rel-10 terminal can receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. The LTE Rel-10 standard (3GPP TS 36.300, V10.12.0) supports up to 5 aggregated CCs where each CC is limited in the RF specifications to have one of six bandwidths namely 6, 15, 25, 50, 75 or 100 RB (corresponding to 1.4, 3 5 10 15 and 20 MHz respectively).
The number of aggregated CCs as well as the bandwidth of the individual CCs may be different for UL and DL. A symmetric configuration refers to the case where the number of CCs in DL and UL is the same whereas an asymmetric configuration refers to the case that the number of CCs is different in DL and UL. It is noted that the number of CCs configured in the network may be different from the number of CCs seen by a terminal. A terminal may for example support more DL CCs than uplink CCs, even though the network offers the same number of UL and DL CCs.
CCs are also referred to as cells or serving cells. More specifically, in an LTE network the cells aggregated by a terminal are denoted PCell and SCell. The term serving cell comprises both PCell and SCells. All UEs have one PCell and which cell is a UEs PCell is terminal specific and is considered “more important”, i.e. vital control signaling and other important signaling is typically handled via the PCell. UL control signaling is always sent on a UEs PCell. The CC configured as the PCell is the primary CC whereas all other component carriers are secondary serving cells. The UE/terminal can send and receive data both on the PCell and SCells. For control signaling such as scheduling commands this could either be configured to only be transmitted and received on the PCell but where the commands are also valid for SCell, or it can be configured to be transmitted and received on both PCell and SCells. Regardless of the mode of operation, the UE/terminal will only need to read the broadcast channel in order to acquire system information parameters on the PCC. System information related to the SCCs may be provided to the UE/terminal in dedicated RRC messages.
During an initial access a LTE Rel-10 terminal (UE) behaves similar to a LTE Rel-8 terminal (UE). However, upon successful connection to the network a Rel-10 terminal (UE) may—depending on its own capabilities and the network—be configured with additional serving cells in the UL and DL. Configuration is based on RRC. Due to the heavy signaling and rather slow speed of RRC signaling it is envisioned that a terminal may be configured with multiple serving cells even though not all of them are currently used.
LTE CA supports efficient use of multiple carriers, allowing data to be sent/received over all carriers. There is support for cross-carrier scheduling avoiding the need that the UE/terminal listens to all carrier-scheduling channels at all times. The solution relies on a tight time synchronization between the carriers.
CA requires tight synchronization between the PCell and the SCell, thus essentially requiring a common location for the antenna(s) or very low latency backhaul connections between them. To enable similar benefits as in CA also for cases where different base stations and/or antenna sites are used with relaxed backhaul latency requirements, 3GPP initiated and introduced the concept called dual connectivity.
Dual connectivity is a solution currently being standardized by 3GPP to support UEs connecting to multiple carriers to send/receive data on multiple carriers at the same time. Below is an overview description taken from 3GPP TS 36.300 V13.3.0: E-UTRAN supports Dual Connectivity (DC) operation whereby a multiple RX/TX UE in a RRC_CONNECTED mode is configured to utilize radio resources provided by two distinct schedulers, located in two eNBs (base stations) connected via a non-ideal backhaul over the X2 interface. eNBs involved in dual connectivity for a certain UE may assume two different roles: an eNB may either act as a MeNB or as an SeNB. In dual connectivity a UE is connected to one MeNB and one SeNB. DC also makes it possible to send and/or receive data over all LTE carriers, without requiring tight time synchronization as in CA. This is enabled because the UE will listen to all scheduling channels on all carriers.
FIG. 3 schematically illustrates the radio user plane protocol architecture for DC. In DC, the radio protocol architecture that a particular bearer uses depends on how the bearer is setup. Three alternatives exist, MCG bearer, SCG bearer and split bearer. Those three alternatives are depicted on FIG. 3. In particular, the left side of FIG. 3 shows the user plane flow for MCG bearers arriving from the core network to the MeNB and MeNB handling this flow without SeNB involvement. The middle flow in FIG. 3 arriving from the core network to the MeNB shows the case of split bearers. In this case the received user plane flow can be handled by both the MeNB and the SeNB. The right side of FIG. 3 shows the user plane arriving to the SeNB for SCG bearers and SeNB handling this flow. SRBs are always of the MCG bearer and therefore only use the radio resources provided by the MeNB. It is further noted that DC may also be described as having at least one bearer configured to use radio resources provided by the SeNB.
Inter-eNB control plane signaling for dual connectivity is performed by means of X2 interface signaling via the X2 network interface, as shown in FIG. 3. Further, control plane signaling towards the MME is performed by means of S1 interface signaling, as further illustrated in FIG. 4A. There may be only one S1-MME connection per UE between the MeNB and the MME. Each eNB should be able to handle UEs independently, i.e. provide the PCell to some UEs while providing SCell(s) for SCG to others. Each eNB involved in dual connectivity for a certain UE owns its radio resources and is primarily responsible for allocating radio resources of its cells. Any required coordination between MeNB and SeNB is performed by means of the X2 interface signaling. FIG. 4A shows C-plane (control plane) connectivity of eNBs involved in dual connectivity for a certain UE, and the MeNB is C-plane connected to the MME via S1-MME, the MeNB and the SeNB are interconnected via X2-C.
FIG. 4B schematically illustrates U-plane (user plane) connectivity of eNBs involved in dual connectivity for a certain UE. Here, the U-plane connectivity may depend on the bearer option configured. For MCG bearers, the MeNB is U-plane connected to the S-GW via S1-U and the SeNB is not involved in the transport of user plane data. For split bearers, on the other hand, the MeNB is U-plane connected to the S-GW via S1-U and in addition, the MeNB and the SeNB are interconnected via X2-U. For SCG bearers, the SeNB is directly connected with the S-GW via S1-U.